1. Field of the Invention
The present invention relates to a laser processing apparatus, in particular, a laser processing apparatus which modifies a semiconductor sample. In addition, the present invention relates to a method for manufacturing a semiconductor substrate which can achieve modification of a semiconductor sample by laser irradiation.
2. Description of the Related Art
In recent years, instead of a bulk silicon wafer, integrated circuits using an SOI (silicon on insulator) substrate have been developed. By utilizing characteristics of a thin single crystal silicon layer formed over an insulating layer, a semiconductor layer of a transistor formed in the integrated circuit can be electrically separated from each other completely. Further, since the fully depleted transistors can be formed, a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
Known examples of SOI substrates are SIMOX substrates and bonded substrates. For example, for an SOI structure of SIMOX substrate, oxygen ions are implanted into a single crystal silicon substrate and a heat treatment at 1300° C. or higher is performed to form a buried oxide (BOX) layer, so that a single crystal silicon thin film is formed on the surface.
An SOI structure of bonded substrates is obtained by bonding of two single crystal silicon substrates (a base substrate and a bond substrate) to each other with an oxide film interposed therebetween and by thinning of one of the two single crystal silicon substrates (the bond substrate) on a back side (a side which is opposite to a bonding surface), whereby a single crystal silicon thin film is formed. Since it is difficult to form an even and thin single crystal silicon thin film through grinding or polishing, a technique employing hydrogen ion implantation which is called Smart-Cut® has been proposed (see, for example, Reference 1: Japanese Published Patent Application No. H 5-211128).
A summary of a method for manufacturing this SOI substrate is described. By implantation of hydrogen ions into a silicon wafer, an ion implantation layer is formed at a predetermined depth from the surface. Next, a silicon oxide film is formed by oxidizing another silicon wafer which serves as a base substrate. After that, the silicon wafer into which hydrogen ions are implanted is bonded to the silicon oxide film of another silicon wafer, so that the two silicon wafers are attached to each other. Then, through a heat treatment, cleavage is caused in the silicon wafer, using the ion implantation layer as a cleavage plane, whereby a substrate in which a thin single crystal silicon layer is bonded to the base substrate is formed.
In addition, a method for forming an SOI substrate in which a single crystal silicon layer is bonded to a glass substrate is known (see, for example, Reference 2: Japanese Published Patent Application No. H11-097379). In Reference 2, a separation surface is mechanically polished to remove a defective layer which is formed through hydrogen ion implantation or a step of several to several tens of nanometers on the separation surface.
Methods for manufacturing a semiconductor device in which a highly heat-resistant substrate is used as a supporting substrate, utilizing Smart Cut®, are disclosed in Patent Document 3: Japanese Published Patent Application No. H11-163363 and Patent Document 4: Japanese Published Patent Application No. 2000-012864, and a method for manufacturing a semiconductor device in which a light-transmitting substrate is used as a supporting substrate, utilizing Smart Cut®, is disclosed in Patent Document 5: Japanese Published Patent Application No. 2000-150905.
On the other hand, as a method for performing a heat treatment on a silicon thin film over a substrate, a laser annealing apparatus for processing a laser beam and a microwave by being combined has been known (see, for example, Patent Document 6: Japanese Published Patent Application No. 2001-223175). Patent Document 6 discloses a method in which, by irradiating silicon melted by a laser beam with a microwave, cooling time is prolonged more than the case where silicon is melted only by a laser beam and expansion of silicon crystallization depth can be achieved.
In addition, as a method for performing a heat treatment on a silicon thin film over another substrate, a laser annealing apparatus for processing a laser beam, a microwave, and heated nitrogen by being combined has been known (see, for example, Patent Document 7: Japanese Published Patent Application No. S58-70536). In Patent Document 7, the substrate is irradiated with heated nitrogen and the microwave from the back side thereof, and the substrate is irradiated with the laser beam in a heated state.